Monday, January 19, 2009
Just the taste of a sweet beverage can result in liking, wanting and sipping of more of the sweet beverage. (Please see the previous blogpost for details.) The liking-wanting-sipping phenomenon happens even when the sweet beverage is not actually consumed, and happens in less than a minute. What causes this reaction to the taste of sweet?
In 1989, L.H. Schneider observed that dopamine receptors in the brain are stimulated when rats are allowed to feed themselves sweet solutions. Since that time, many investigators have noticed a relationship between either the taste of sweet or the actual consumption of sweet and the response of dopamine receptors in the brain, both in rats and in humans.
Dopamine is a neurotransmitter found in the midbrain. It has many functions, but one of them is the ability to produce prolonged feelings of pleasure. Increased dopamine signaling is involved in the mechanism of addiction to cocaine, amphetamines, and nicotine. At a more moderate level, novel foods, sweet foods, and overeating also cause increases in brain dopamine.
In order to investigate possible rebound effects of overstimulation with dopamine followed by abrupt dopamine withdrawal, a group of rats was treated for five days with l-dopa (a substance which is converted to dopamine in the brain). The research is described in an article in the December 2008 issue of Nutrition & Metabolism. After treatment was completed, the rats in the previously-treated l-dopa group were compared with an untreated control group. Over the next 12 weeks both groups of rats were allowed to eat as much food as they desired. At the end of that time, the previously-treated rats had gained 15% more weight than the control group.
Why did this happen? The authors hypothesize that treatment with the dopamine precursor l-dopa caused overstimulation of the dopamine signaling system in the rats. This, in turn, caused downregulation of dopamine receptors and decreased endogenous dopamine production. When the l-dopa treatment ceased, the rats were left with few dopamine receptors and low endogenous dopamine production. To compensate for this, the rats used the mechanism of overeating to compensate for their relative dopaminergic deficiency.
Rats are not humans. Nevertheless, it is possible to suggest that eating or tasting sweet food causes an overstimulation of the dopamine signaling system. This produces a downregulation of dopamine signaling such that, in the absence of sweet, there is a noticeable decrease in energy, motivation and mood. Sweets are legal, cheap and easy to obtain. If the sweet-eater wishes to experience the pleasant feelings associated with dopamine overstimulation, it will be very easy to continue eating sweets. And if he is unable to wait several days to a week to allow the body to return to its normal level of dopamine production and receptor activity, it will be harder than he might have expected to eliminate sweet tastes from his diet.